SAW technology has found a number of applications in the electronics and RF art. Due to the fact that SAW wavelengths are typically 10.sup.5 times shorter than that of electromagnetic waves having a corresponding frequency, SAW technology has found particular applications where miniaturisation is important or desirable. One such application is the use of SAW filters in radio telephones where the typically small size and weight of SAW filters is highly advantageous over conventional technologies, such as ceramic filters, dielectric filters, and filters using magnetostatic waves. Generally, it is a requirement of such SAW filters that they have low-loss, typically insertion losses of 1.about.3 dB for RF use, although for IF filters somewhat higher insertion loss of 5.about.13 dB could be acceptable. Additionally, it is desirable that SAW filters have a good shape factor and high suppression levels in the stopband.
A typical example of a conventional SAW filter is a SAW filter in which SAW energy is transferred between two spaced apart interdigital transducers (IDTs). The IDTs comprise two sets of equally spaced metal strips (electrode fingers), which are formed on the surface of a piezoelectric substrate. The electrode fingers in each set are typically electrically coupled together by bus-bars and are interleaved (interdigitated) with the electrode fingers of the other set. This arrangement can generate SAWs in both directions transverse to each electrode finger when a high frequency electrical signal is applied between the sets of electrode fingers, and it can also generate an electrical voltage when SAWs are incident on the electrode fingers. These processes are most efficient when the frequency of the SAWs is such that the periodicity of the electrode fingers in each set is close to or the same as the SAW wavelength, or some multiple of this frequency, In the simplest form of IDT, the spacing between adjacent electrode fingers of a set of electrode fingers is one SAW centre wavelength, i.e. one electrode finger per period in each set of electrodes. Thus there are two electrode fingers per period taking into account both sets of electrode fingers. The conventional terms in the art for such arrangements are "two electrodes per period" or "two electrodes per wavelength". However, it is possible to have more than one finger per SAW wavelength (period).
A particular path for a surface acoustic wave comprising SAW elements such as IDTs and/or reflection gratings is known as a track.
For conventional filters, to a first order the lengths of the transducers substantially determine the bandwidth of the filter since BW .alpha.1/L where L is the length of the transducers. Thus, the more narrow the bandwidth requirement the longer the transducers need to be. Increasing the length of transducers to decrease the bandwidth of the filter mitigates against advantages of small size usually associated with SAW devices. In particular, there has recently been a requirement for narrow-band filters for portable apparatus, and it is desirable to be able to provide such narrow-band devices without any increase in the size of the components and preferably with a decrease in the size of the components relative to conventional components and known SAW devices.
A known SAW filter device is described in a paper entitled "A New Compact SAW Low Loss Filter for Mobile Radio" presented at the seventh European Frequency and Time Forum held at Neuchatel, Switzerland 16-18 Mar. 1993 and is disclosed in International Patent Application WO 93/08641. In this known device there are two tracks each having an input and output Single Phase Uni-Directional Transducer (SPUDT) separated by a reflection grating. The difference in direct path lengths between the input and output SPUDTs for respective tracks is half a SAW wavelength (.lambda./2). The two input SPUDTs are electrically coupled together in parallel and the two output SPUDTs are electrically coupled together in parallel. Due to the half wavelength difference in separation, voltages generated at respective output SPUDTs by directly coupled SAWs launched by respective input SPUDTs are .pi. out of phase at the centre frequency and cancel each other and thus no output voltage is generated across the output SPUDTs. By a complex arrangement of partial reflections from reflection gratings disposed in respective tracks, SAWs in respective tracks undergoing such reflections are coherently incident at respective output SPUDTs and the individual voltages generated by each SPUDT add in phase to provide an output. By virtue of the reflected paths for the SAWs the length of the impulse response for the input and output SPUDTs is effectively lengthened. Thus, a device having a particular bandwidth may be fitted into a smaller space than previously possible. Alternatively, a device having a narrower bandwidth than before may fit into the same space. However, the known device has some drawbacks. Exact cancellation of the directly coupled surface acoustic waves only occurs at the designed centre frequency (f.sub.o) of the device. At frequencies outside of this centre frequency f.sub.o the suppression in the stopband is progressively reduced as the frequency progressively deviates from the centre frequency f.sub.o. The suppression can be as poor as 10 dB at a 20% deviation from the centre frequency f.sub.o of the device. Thus, the frequency selectivity of the device is impaired. Additionally, parasitic signals such as bulk wave signals are only cancelled at the centre frequency. Furthermore, other parasitic signals such as electromagnetic or capacitative coupling between respective input and output IDTs are not compensated by the known device.